[ADT] Add C++20-style llvm::identity_cxx20 #164143
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Currently, our llvm::identity<T> is not quite like std::identity from C++20. Ours is a template struct while std::identity is a non-template struct with templatized operator(). This difference means that we cannot mechanically replace llvm::identity with std::identity when we switch to C++20 in the future. This patch implements llvm::identity_cxx20, which behaves exactly like std::identity. Once this patch lands, I'm planning to migrate users of llvm::identity to the new style. There aren't that many: - 4 instances of SparseSet<...> - 3 instances of SparseMultiSet<...> - about 50 instances of IndexedMap<...>
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@llvm/pr-subscribers-llvm-adt Author: Kazu Hirata (kazutakahirata) ChangesCurrently, our llvm::identity<T> is not quite like std::identity from This patch implements llvm::identity_cxx20, which behaves exactly like Once this patch lands, I'm planning to migrate users of llvm::identity
Full diff: https://github.com/llvm/llvm-project/pull/164143.diff 2 Files Affected:
diff --git a/llvm/include/llvm/ADT/STLForwardCompat.h b/llvm/include/llvm/ADT/STLForwardCompat.h
index 273a5cf8aa8ff..0e9bd2d455965 100644
--- a/llvm/include/llvm/ADT/STLForwardCompat.h
+++ b/llvm/include/llvm/ADT/STLForwardCompat.h
@@ -19,6 +19,7 @@
#include <optional>
#include <type_traits>
+#include <utility>
namespace llvm {
@@ -117,6 +118,15 @@ struct detector<std::void_t<Op<Args...>>, Op, Args...> {
template <template <class...> class Op, class... Args>
using is_detected = typename detail::detector<void, Op, Args...>::value_t;
+struct identity_cxx20 // NOLINT(readability-identifier-naming)
+{
+ using is_transparent = void;
+
+ template <typename T> constexpr T &&operator()(T &&self) const noexcept {
+ return std::forward<T>(self);
+ }
+};
+
//===----------------------------------------------------------------------===//
// Features from C++23
//===----------------------------------------------------------------------===//
diff --git a/llvm/unittests/ADT/STLForwardCompatTest.cpp b/llvm/unittests/ADT/STLForwardCompatTest.cpp
index 4a8f53cf72f94..2a97e8d6a552f 100644
--- a/llvm/unittests/ADT/STLForwardCompatTest.cpp
+++ b/llvm/unittests/ADT/STLForwardCompatTest.cpp
@@ -184,4 +184,26 @@ TEST(TransformTest, ToUnderlying) {
static_assert(llvm::to_underlying(E3::B3) == 0);
}
+TEST(STLForwardCompatTest, IdentityCxx20) {
+ llvm::identity_cxx20 identity;
+
+ // Test with an lvalue.
+ int X = 42;
+ int &Y = identity(X);
+ EXPECT_EQ(&X, &Y);
+
+ // Test with a const lvalue.
+ const int CX = 10;
+ const int &CY = identity(CX);
+ EXPECT_EQ(&CX, &CY);
+
+ // Test with an rvalue.
+ EXPECT_EQ(identity(123), 123);
+
+ // Test perfect forwarding.
+ static_assert(std::is_same_v<int &, decltype(identity(X))>);
+ static_assert(std::is_same_v<const int &, decltype(identity(CX))>);
+ static_assert(std::is_same_v<int &&, decltype(identity(int(5)))>);
+}
+
} // namespace
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kuhar
approved these changes
Oct 19, 2025
Lukacma
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Oct 29, 2025
Currently, our llvm::identity<T> is not quite like std::identity from C++20. Ours is a template struct while std::identity is a non-template struct with templatized operator(). This difference means that we cannot mechanically replace llvm::identity with std::identity when we switch to C++20 in the future. This patch implements llvm::identity_cxx20, which behaves exactly like std::identity. Once this patch lands, I'm planning to migrate users of llvm::identity to the new style. There aren't that many: - 4 instances of SparseSet<...> - 3 instances of SparseMultiSet<...> - about 50 instances of IndexedMap<...>
aokblast
pushed a commit
to aokblast/llvm-project
that referenced
this pull request
Oct 30, 2025
Currently, our llvm::identity<T> is not quite like std::identity from C++20. Ours is a template struct while std::identity is a non-template struct with templatized operator(). This difference means that we cannot mechanically replace llvm::identity with std::identity when we switch to C++20 in the future. This patch implements llvm::identity_cxx20, which behaves exactly like std::identity. Once this patch lands, I'm planning to migrate users of llvm::identity to the new style. There aren't that many: - 4 instances of SparseSet<...> - 3 instances of SparseMultiSet<...> - about 50 instances of IndexedMap<...>
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Currently, our llvm::identity is not quite like std::identity from
C++20. Ours is a template struct while std::identity is a
non-template struct with templatized operator(). This difference
means that we cannot mechanically replace llvm::identity with
std::identity when we switch to C++20 in the future.
This patch implements llvm::identity_cxx20, which behaves exactly like
std::identity.
Once this patch lands, I'm planning to migrate users of llvm::identity
to the new style. There aren't that many: